1. Field of the Invention
The present invention relates to a magneto-optical recording medium or the like which undergoes information recording/reproduction by a laser beam using a magneto-optical effect and, more particularly, a method of reproducing information from a magneto-optical recording medium, that can attain high-density recording of a medium, and an information reproduction apparatus using the magneto-optical recording medium.
2. Related Background Art
As a rewritable, high-density recording scheme, a magneto-optical recording medium that records information by writing domains in a magnetic thin film using heat energy of a semiconductor laser, and reads out that information using a magneto-optical effect has become popular. In recent years, demand has arisen for a large-capacity recording medium by increasing the recording density of such magneto-optical recording medium.
The line recording density of an optical disk such as a magneto-optical recording medium largely depends on the laser wavelength of a reproduction optical system and the numerical aperture of an objective lens. That is, since the beam waist diameter is determined upon determination of the laser wavelength .lambda. of the reproduction optical system and the numerical aperture NA of the objective lens, the spatial frequency that can be detected upon reproducing a signal is limited to about 2NA/.lambda.. Therefore, in order to realize high-density recording in a conventional optical disk, the laser wavelength of the reproduction optical system must be shortened, or the numerical aperture NA of the objective lens must be increased. However, a decrease in laser wavelength or an increase in numerical aperture of the objective lens has limitations. For this reason, a technique for increasing the recording density by modifying the structure of the recording medium or its read method has been developed.
For example, Japanese Laid-Open Patent Application No. 6-290496 proposed a method of reproducing a high-density recording signal by making the domain wall present at the boundary of neighboring recording marks move in the presence of a temperature gradient, and detecting the wall motion. According to this method, the wall moves at the instant when the recording medium has been heated to the mobility temperature of the wall, and a high-density recording signal is reproduced by detecting such wall motion. Hence, a signal can be reproduced independently of the diffraction limit of light.
However, the magneto-optical reproduction method described in Japanese Laid-Open Patent Application No. 6-290496 does not consider any measure against noise arising from the influences of a domain wall from behind the beam spot (with respect to the traveling direction).